How to filter other website links in html in php (domain name whitelist function)_PHP tutorial

* Filter external links
* @param array $local_domain Domain name array of this site
* @param string $message Text content
*/
    function replace_outer_links($local_domain_arr, $message) {

        $pattern= '/<[^>]*href=['"]http[s]?://(?!' ;
        $i = 0 ;
        foreach ($local_domain_arr as $local_domain){
            if($i==0){
                 $pattern .= 'www.' .$local_domain.'|'.$local_domain.'|[w_]+.'.$local_domain ;
            }else{
                 $pattern .= '|www.' .$local_domain.'|'.$local_domain.'|[w_]+.'.$local_domain ;
            }
             $i++ ;
        }
        $pattern .=')[^'^"]*['"][^>]*>(.+?)/is';  
        return preg_replace($pattern,'$1',$message);
    }